1. Field of the Invention
The present invention relates to a method for decoding a code division multiplex signal using a code selected from various codes available for decoding, based on comparison with the code division multiplex signal.
2. Description of the Related Art
The code division multiplex access (CDMA) technique is a multiple access method in which all subscribers of the appropriate mobile radio system are permitted to make simultaneous use of the entire available system bandwidth. To be able to avoid collisions between the individual subscribers, the individual subscriber signals are provided with different code sequences which thereby permits a unique assignment of the received signals. Spreading of the individual signals occurs during this process, as a result of which the bandwidth is multiplied, for which reason this technique is also denoted as a spread spectrum technique. The receiver receives this signal, demodulates it and carries out despreading, use being made of a spread signal which is synchronous with the transmitter. The transmitter admittedly receives not only the desired signal of the transmitter, but also additional signals from other transmitters, which transmit in the same frequency band. However, the despreading operation ensures that it is only the signal which uses the same and synchronous spreading code as the receiver which is despread and at its bandwidth reduced.
It is therefore important to carry out synchronization between the transmitter and the receiver in CDMA systems. As a rule, this is performed in several steps using control information transmitted specifically for this purpose, frequent synchronization initially being carried out, followed by temporal synchronization. This requires carrying out in the receiver code synchronization or code identification in order to determine that spreading and scrambling code with the aid of which the transmitter signal has been encoded. Only then is it possible to despread or decode the received signal.
In order to find the correct code for the decoding operation, all available codes must be correlated at the receiver end with the received signal, the code to be used then being selected as that one whose code sequence has the greatest correspondence or similarity with the received signal. That is to say, the following sum is formed for each possible code sequence ci with reference to the received signal sequence r(t):       ∑          t      =      0        T    ⁢            r      ⁡              (        t        )              ·                  c        i            ⁡              (        t        )            
Here, T denotes the correlation time interval over which the multiplication results between the individual values of the received signal sequence r(t) and the code sequence ci(t) are summed or integrated.
As an example, 16 possible codes are provided, that is to say i=0 . . . 15, for the UMTS (Universal Mobil Telecommunication System) mobile radio standard. The results calculated in accordance with the above formula for the individual code sequences ci(t) are compared, and that code sequence ci(t) with the maximum energy value, that is to say with the greatest correspondence or similarity with the received sequence r(t), is determined. It can then be assumed that the transmitted signal has been encoded with the aid of this code so that subsequently decoding is carried out with the aid of this code.
It may be seen with the aid of the above formula that for n different codes it is necessary in general to carry out n correlations with a corresponding number of multiplications. This can be very complicated, and requires a correspondingly high outlay on hardware for adders and multipliers in the receiver.